Dark Matter Mysteries Darken

Four recent astronomical observations have cast doubt on scientists’ prior claims and assumptions.

Physicists believe that 83% of all the matter in our universe is invisible and very different from the matter in our bodies, in planets, in stars, or in anything else we can see. We call this strange stuff “dark matter.” It emits no light, it casts no shadow, and its only effect is to exert an immense gravitational pull on everything that we do see. Dark matter may be the greatest mystery in physics; we have almost no solid evidence to indicate what it is. The most popular guess is that dark matter consists of WIMPs, Weakly Interacting Massive Particles, which sounds much more professional than AGAC (Ain’t Got A Clue).

This is what the recent observations tell us…

Dark Matter Is Heavy

Three years of observations by the Fermi Gamma-ray Space Telescope have set a new stringent limit on the WIMP mass — it must be greater than 40 GeV, about the mass of a calcium atom. This is not an unreasonable value. Particles of normal matter have masses up to 170 GeV. But this new limit does contradict three experiments done deep below Earth’s surface that reported faint indications of dark matter with masses from 7 to 12 GeV.

Physicists from Brown University and the Fermi-LAT collaboration searched for gamma rays coming from seven dwarf galaxies from the annihilation of WIMPs with anti-WIMPs. The selected dwarf galaxies appear to have lots of dark matter and lack any high-energy activity due to normal matter, such as supernovae. Because the normal matter in these galaxies generates so few gamma rays (what physicists call “low background noise”), these researchers were able to set the best limit yet on the WIMP mass.

No Dark Matter In Our Backyard

European astronomers report finding almost no dark matter within 80,000 trillion miles of Earth. If true, this may doom all Earth-based laboratory experiments designed to catch and analyze dark matter particles.

Prior measurements of the velocities of stars near the edge of our galaxy, the Milky Way, indicated that our galaxy is embedded in a vast halo of dark matter. These stars are moving so fast that the gravity of all the galaxy’s visible matter isn’t enough to keep them from flying off into the great beyond. But, recent measurements of the movements of over 400 stars near our Sun tell a different story. Using observations from La Sillia in Chile and other telescopes, astronomers computed the amount of mass within 13,000 light-years of our Sun. That amount matched quite precisely the total mass of the normal matter (stars, dust and gas) observed in this region, leaving no extra mass to attribute to dark matter.

Prior estimates of the amount of dark matter in the Milky Way and assumptions about the halo shape, predict 700 grams of dark matter in each volume the size of Earth. But, when astronomers subtracted the amount of normal matter from the amount of total matter, the amount left for dark matter was 0±70 grams. Note again that this new measurement applies only to the vicinity of our Sun.

A possible explanation is that the dark matter halo has a radically different shape than predicted by present theories.

There are over 50 major Earth-bound experiments currently searching for dark matter. They all assume Earth is passing through the Milky Way’s vast cloud of dark matter and that eventually some dark matter particles will hit our detectors. If Earth is indeed in a hole in that dark matter cloud, we would have nothing here to detect.

Dark Matter Not Centered In Galaxy Centers

Recent Hubble Space Telescope observations confound yet another popular assumption: that dark matter clumps at the center of galaxies. Since dark matter is very massive, some expected it to be pulled into galactic cores — the heaviest things usually fall to the bottom of a gravitational potential.

Apparently that doesn’t happen.

This study was also done on nearby dwarf galaxies: Fornax and Sculptor. These galaxies contain only about 1% as many stars as our Milky Way. But up to 99% of the mass of a dwarf galaxy may be in the form of dark matter. The researchers measured the motions of 2000 stars and used these to compute the distribution of dark matter. They found no substantial peaking at the galaxy centers; the dark matter seems uniformly spread over a vast area. This will send a lot of theorists back to their whiteboards.

Dark Matter Might be Sticky After All

Another Hubble Space Telescope study seems to contradict yet more common wisdom about dark matter. These data show that dark matter might have some interaction other than just gravity – that perhaps it can stick to itself.

Prior analysis, based on the spectacular image of the Bullet Cluster, shown below, indicated that dark matter does not interact with normal matter, neutral or charged, and does not interact with itself, except via gravity. In this image, two galaxy clusters, each with 1000 trillion stars, passed through one another 150 million years ago. Here the computed dark matter halos are shown in purple. The key point is that each halo passed through galaxies, clouds of charged normal matter (shown in pink), and even the other dark matter halo with no apparent effect.

Yet in the new Hubble image of another galaxy cluster collision, shown below, a very different picture emerges. In this image, the computed dark matter locations, shown in teal, lag behind their clusters. This may indicate a substantial dark matter interaction, possibly even that the dark matter halos stuck to one another.

There might be alternative explanations for any individual celestial image — our universe is so vast that there are many strange events to ponder.

For those seeking definitive answers, these recent developments may be discouraging. They show that much of the little that we thought we knew about dark matter is either wrong or needs major revision.

But for those who enjoy exploring the mystery of our universe, dark matter has gotten a lot more interesting.